1. Field of the Invention
This invention related to optical information sensing and, more particularly, to circuits and processes for attenuating smearing of video information due to leakage between individual light sensing elements and elements contiguously and proximally positioned around each of the elements.
2. Description of the Art
The total energy received by charge coupled device (CCD) image sensors can cause leakage or contamination in the horizontal direction only for a single array of CCD elements. When the CCD elements are arranged in the rows and columns, the leakage will be in both horizontal and vertical directions. As used herein, "leakage" means the contribution to the contamination of an electrical signal generated by one CCD element in response to energy impingement on the CCD, caused by energy transferred to the CCD elements from neighboring CCD elements due to either horizontal spreading of bright long-wavelength signals or the mixing of charges from neighboring elements during transfer of the charges to a shift register. The leakage is the greatest from immediately adjacent neighboring elements, and decreases rather rapidly as the distance increases between the element being read and the neighboring elements contributing to the leakage.
Currently available circuits for enhancing optical imaging of CCD arrays typically endeavor to improve performance of the array, but have traditionally limited signal processing to individual columns of CCD elements. See, for example, U.S. Pat. No. 4,345,148 to Pines, et al. Early efforts to reduce smear occurring during interline charge transfer included generator of a signal pattern representing the smear charge signal pattern present in one line of a field and subsequent subtraction of the signal pattern from the line of charge signals being read, as taught by Levine, U.S. Pat. No. 4,010,319. More current efforts such as suggested by May, U.S. Pat. No. 4,736,439 use pre-processing circuits to obtain median values for a matrix of one or more lines of predetermined pixel neighborhoods, and then subtract the median values read from individual pixels, while Nagumo, U.S. Pat. No. 4,558,366, includes the derivation of signals of "n" horizontal lines from signals of "N" horizontal lines of an image sensing array, where "n" is greater than "N," whereby signals of the "n" horizontal lines lack inherent accumulation of signal components due to picked-up image light and represent only undesirable signal components mixed during vertical transfer of the charge. A compensating circuit reduces the undesirable signal caused by the extraneous undesired signal components. Generally however, such schemes are effective only to compensate for low and high frequency extraneous signal components.
In charge-injection-device (CID) imaging systems, a portion of injected charge moves into laterally adjacent pixels, a phenomenon which increases as the pixels density of the CID imaging system increases. Vogelsong, U.S. Pat. No. 4,768,098 suggests reducing cross-talk in a two-dimensional array of pixels by storing as a first video signal charges sequentially read from each pixels in a row, storing charges read from each pixels in the second row, injecting the charges stored as the first video signal in all of the pixels of the first row, and then sequentially re-reading the stored charge from each of the pixels of the first row as a second video signal. The second video signal is subtracted from first video signal, ostensibly to provide an output signal substantially free of injected cross-talk. Vogelsong '098 denigrates those "techniques which reduce cross-talk effect by mathematically performing cross-talk inversion" as suffering from a drawback requiring knowledge of "the exact amount of cross-talk of each location." Vogelsong, '098, column 1, beginning with line 36.
Other recent circuits such as Ozawa, et al., U.S. Pat. No. 4,543,610 alternately read odd-numbered and even-numbered horizontal rows of picture elements and by subsequent subtraction, endeavor to remove vertical smear attributable to the solid-state imaging elements. Powell, U.S. Pat. No. 4,442,454 combines block overlap transform and a multi-stage procedure using a window moved pixel-by-pixel, row-wise and column-wise over an entire image, and obtains final image values for each pixel by averaging values derived from each of the overlapping windows. Powell U.S. Pat. No. 4,805,031 advocates the grouping of pixel image values corresponding to pixels aligned along predetermined directions, processes groups of pixel image values using a transformed algorithm and a thresholding operation to separate image components from noise components for each of the groups of pixel image values, and repeatedly accumulates and averages image value components from every group. The successive steps however, amount to a correction of earlier corrections which deleteriously multiply, and thereby contaminate previous error corrections, because once the data is subjected to an initial processing, the contamination begins to spread among additional units of data read from other imaging elements. Correction or compensation for contamination after the initial processing is difficult, and with currently available techniques, only approximate. Moreover, spread of contamination during processing prevents reliable correction of sequential smear contamination of image signals.